1. Field of the Invention
The present invention relates generally to a hydraulic power transmission joint for use in 4 wheel-drive motor vehicles for the distribution of driving forces between front and rear wheels, and more particularly to a hydraulic power transmission joint designed to suppress possible sudden torque variations upon the switching of torque transmission characteristics by use of a centrifugal torque varying mechanism.
2. Description of the Related Arts
Such a conventional hydraulic power transmission joint is known from U.S. Pat. Nos. 5,706,658 and 5,983,635.
This hydraulic power transmission joint comprises a housing coupled to one of input and output shafts that are capable of relative rotations and having a cam face formed on its inner side surface; a rotor coupled to the other of said input and output shaft and being rotatably accommodated in the housing, the rotor having a plurality of axially extending plunger chambers; a plurality of plungers each being reciprocatively accommodated in each of the plurality of plungers under a biasing force of return springs, the plurality of plungers being operated by the cam face upon the relative rotations of the input and output shafts; a discharge hole formed in the rotor and leading to the plurality of plunger chambers; and an orifice having a high-pressure chamber that leads to the discharge hole, the orifice generating a flow resistance under the action of flow of oil discharged by operations of the plurality of plungers.
In the hydraulic power transmission joint being currently developed by the present inventors, a valve block is coupled to the rotor for rotations jointly therewith and is provided with a centrifugal torque varying mechanism. The centrifugal torque varying mechanism has a weight which when the vehicle velocity exceeds a predetermined level, pivots around a weight fulcrum by a centrifugal force to open a relief hole that has been blocked by a check valve so far, thereby achieving a release of the high-pressure oil. Switching is thus made to a lower torque transmission characteristic than the torque transmission characteristic used for the duration in which the relief hole is closed, to thereby prevent the temperature of the joint from raising due to the increased vehicle velocity.
Referring to FIG. 1, there is shown by way of example a centrifugal torque varying mechanism comprising a valve block 101 that includes a relief hole 102 leading to a high-pressure chamber. When the vehicle velocity V exceeds a predetermined vehicle velocity Vt as seen in FIG. 3, a weight can pivot by a centrifugal force so that a member 104 for receiving a check ball 103 is displaced to the direction of an arrow E, allowing the check ball 103 to open the relief hole 102. At that time, oil is released through the relief hole 102 as indicated by an arrow F. A large variation occurs in the area of flow passage when the relief hole 102 is opened by the check ball 103, and as indicated by an arrow J1 of FIG. 3 the torque transmission characteristic is switched from a first torque transmission characteristic H to a lower second torque transmission characteristic I, resulting in a large torque reduction rate. When the vehicle velocity V drops to below the predetermined vehicle velocity Vt and the weight returns to its original position after the switching to the second torque transmission characteristic I the member 104 receiving the check ball 103 is displaced to the direction of an arrow G as seen in FIG. 2, allowing the check ball 103 to block the relief hole 102. The torque transmission characteristic upon this switching as indicated by an arrow J2 of FIG. 3 is switched from the second torque transmission characteristic I to the initial first torque transmission characteristic H, resulting in a large torque increase rate.
In case of such a hydraulic power transmission joint, however, the area of flow passage changes to a large extent when the relief hole is opened or closed by the check ball, with the result that the torque transmission characteristics are liable to be influenced by the variation of flow rate and the torque transmission characteristics may vary in a brief period of time, thus disadvantageously affecting the vehicle behaviors. Furthermore, the switching of the torque transmission characteristics is effected using the predetermined vehicle velocity Vt as the reference value, and hence if the vehicle velocity is in the vicinity of the vehicle velocity Vt, then even a slight variation may induce a switching of the torque transmission characteristics in spite of the travelling at a fixed velocity, which will also affect the vehicle behaviors.
The present invention provides a hydraulic power transmission joint capable of reducing the torque variation rate upon the switching of torque transmission characteristics, preventing frequent switching of the torque transmission characteristics when travelling at an unvarying velocity, and alleviating influences on the vehicle behaviors.
According to an aspect of the present invention there is provided a hydraulic power transmission joint adapted to be interposed between an input shaft and an output shaft that are rotatable relative to each other, to transmit torque as a function of the rotational-speed difference between the input and output shafts, the hydraulic power transmission joint comprising a housing coupled to one of the input and output shafts and having a cam face formed on its inner side surface; a rotor coupled to the other of the input and output shafts and being rotatably accommodated in the housing, the rotor having a plurality of axially extending plunger chambers; a plurality of plungers each being reciprocatively accommodated in each of the plurality of plungers under a biasing force of return springs, the plurality of plungers being operated by the cam face upon the relative rotations of the input and output shafts; an orifice formed in a valve block coupled to the rotor, for generating a flow resistance under the action of flow of oil discharged by operations of the plurality of plungers; and a torque varying mechanism provided in the valve block and having a weight that is pivoted by a centrifugal force to allow a check ball to move to open a relief hole, for the relief of hydraulic pressure, thereby switching a first torque transmission characteristic to a second torque transmission characteristic lower than the first one.
Such a hydraulic power transmission joint of the present invention is characterized in that the torque varying mechanism includes an accommodation hole for receiving the check ball therein, the relief hole being formed in the bottom of the accommodation hole, the accommodation hole being configured such that the area of flow passage between the accommodation hole and the check ball increases linearly as a function of the amount of displacement of the check ball after opening of the relief hole, after which the area of flow passage keeps an unvarying value. By virtue of this, a hysteresis characteristic is obtained in which when a predetermined first vehicle velocity is reached, the weight pivots to allow the check ball to move to open the relief hole, thereby effecting a switching from the first torque transmission characteristic to the second torque transmission characteristic lower than the first one, and in which when the vehicle velocity drops from the first vehicle velocity to the second vehicle velocity lower than the first one, the weight returns to its original position to allow the check ball to close the relief hole, thereby effecting a switching from the second torque transmission characteristic to the first torque transmission characteristic.
The accommodation hole may be flared in section with a plurality of steps from an opening of the relief hole toward an opening of the accommodation hole for example. The accommodation hole may have in section a flared portion with a plurality of steps from the opening of the relief hole toward the opening of the accommodation hole, for example, the accommodation hole further having a straight portion contiguous with the flared portion in the direction of flare.
According to the thus constructed hydraulic power transmission joint of the present invention, a gentler variation in the area of flow passage is achieved relative to the amount of displacement of the check ball induced by the weight provided in the torque varying mechanism, thereby making gentler the variation of the torque transmission characteristic relative to the variation of flow rate. This results in a reduced torque variation rate upon the switching of the torque transmission characteristics, contributing to stabilized vehicle behaviors. Due to the provision of the hysteresis characteristic in which the switching velocity from the first torque transmission characteristic to the second torque transmission characteristic is different from the switching velocity from the second torque transmission characteristic to the first torque transmission characteristic, frequent switching of the torque transmission characteristics upon the low-velocity travelling can be prevented to diminish the influences on the vehicle behaviors.
The above and other objects, aspects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.